The present invention relates to a method and apparatus for the inspection of visual defects on an object under test. The object may be an element coated with a transparent protection layer, such as a thin-film magnetic head used in a magnetic disk unit of a large computer system.
Pertinent conventional techniques applicable to a thin-film magnetic head will be explained. A thin-film magnetic head is fabricated by forming a number of elements on a ceramic wafer (termed simply "wafer" hereinafter) and thereafter separating the wafer into pieces. FIG. 1a is a plan view of an element, and FIG. 1b is a cross-sectional view taken along the line A--A' of FIG. 1a. The element is formed on a wafer 5, and is made up of a coil 1, a magnetic member 2, an insulation layer 4, and a transparent protection layer 3 which covers the components.
The protection layer may contain defects as shown in FIG. 2. A foreign particle 6 is a fragment of impurity which has fallen into and is embedded in the protection layer during the formation of the protection layer for example, and it may be a transparent particle which scatters the light on its surface or an opaque particle which virtually does not reflect the light. A void 8 is a recess which is formed after a foreign particle embedded in the protection layer has been ejected during the polishing process, and an etching remnant 9 is a part of a metallic pattern which has been left unremoved in the etching process.
If metal or other substance on the protection layer of the thin-film magnetic head falls to the disk surface during the operation of the magnetic disk unit, such a serious event as data loss will result due to head crashing. If a foreign particle or metallic etching remnant included in the protection layer appears on the side of a die following die separation, it will cause a crack or erosion by moisture on the protection layer, resulting in a pronounced degradation of reliability of the element. On this account, the protection layer must be inspected for defects thoroughly. However, these defects cannot be detected in terms of the electrical or magnetic characteristics of the element, and therefore visual inspection is indispensable.
A conventional technique for detecting a defect or foreign particle on an electronic circuit pattern is based on the bright field illumination (the light is incident perpendicularly to the object) or dark field illumination (the light is incident obliquely to the object), and the comparison of images of two inspection objects that have intrinsically the same appearance thereby to detect an inconsistent portion as a defect of one object. For example, supposing the presence of an etching remnant 9 on the surface of the protection layer of the coil 1, as shown in FIG. 3a and FIG. 3b which is a cross-sectional view taken along the line C--C' of FIG. 3a, the coil 1 has a rough surface which scatters the light, and therefore the etching remnant 9 is detected as a bright image and the background coil 1 is detected as a dark image under the bright field illumination. Conversely, the etching remnant is detected as a dark image on the bright background under the dark field illumination. In any case, the defect has a high contrast image (the difference of brightness between the defect and its background), and the etching remnant 9 can be detected on the basis of comparison as an inconsistency between images of two elements that should have the same appearance inherently.
An example of conventional techniques for the visual inspection of thin-film magnetic heads is described in the proceeding of "The 4th Symposium on the Industrial Image Sensing Technique", items 106 through 111, sponsored by The Japanese Society for Non-Destructive Inspection, published in 1989, in which the area of pattern of the magnetic member is measured under the bright field illumination, and a defective shape of the magnetic member 2 is detected through the comparison of the measured area with the reference value.
The conventional technique based on the comparison of images of two elements having the same appearance is greatly affected by the element surface characteristics (roughness of the surface) coated by the protection layer. For example, supposing the presence of a transparent foreign particle 6 which reflects the light on its surface on the coil 1, as shown in FIG. 4a and FIG. 4b, which is a cross-sectional view taken along the line d--d' of FIG. 4a, with the coil 1 and foreign particle 6 have the nature of light scattering on the surface. Accordingly, both of the transparent foreign particle 6 and coil 1 are detected as dark images under the bright field illumination, or they are detected as bright images under the dark field illumination. As a result, the transparent foreign particle 6 has a low constrast in the whole image, and its detection will be difficult due to a small difference of element images in the comparative inspection. Another example of difficult defect detection is the presence of an etching remnant 9 on the magnetic member 2, as shown in FIG. 5a and FIG. 5b, which is a cross-sectional view taken along the line B--B' of FIG. 5a. Both the magnetic member 2 and etching remnant 9 have a smooth surface, and they are detected as bright images under the bright field illumination or as dark images under dark field illumination, providing a low contrast defect.
The conventional technique based on the measurement of a pattern area and the comparison of it with the reference value also relies on the premise that the defective portion has a high contrast. Therefore, the technique will not be applicable to low-contrast defects, as mentioned above.
As described above, the conventional techniques do not consider various cases of the material of element and defect in the transparent protection layer and their surface property and optical characteristics, as encountered in the inspection of thin-film magnetic heads. On this account, if a defect which exists in the protection layer resembles the background element surface in the optical characteristics, the defect produces a low-contrast image, making its detection difficult.